The present invention relates to a device for conveying powder, particularly a screw pump advantageously applicable to toner used in an image forming apparatus, a toner conveying device using the screw pump, and a toner filling system.
Some different systems are available for the conveyance of powder and typified by the following systems. A first system includes a coil screw disposed in a piping connecting a powder supply source and a destination. A second system includes a power supply source and a destination arranged one above the other, so that power can be transferred mainly by gravity. A third system uses a screw pump which causes its rotor to rotate in order to move powder in the axial direction of the rotor, thereby feeding the powder from a powder supply source to a destination.
Generally, a copier, printer, facsimile apparatus or similar image forming apparatus uses any one of the above first to third conveying systems for conveying toner. However, a problem with the first system is that the coil screw cannot be surely rotated unless a straight or a gently curved toner transport path, as distinguished from a bent path, is available. Another problem is that an extremely heavy frictional load acts between the coil screw and the piping and increases the drive torque for the coil screw. This not only makes it difficult to convey collected toner over a long distance, but also causes stresses to act on the toner being conveyed and causes it to cohere or melt due to heat.
The second system implements relatively simple toner conveyance. However, collected toner storing means or a developing unit must be arranged substantially integrally with a cleaning unit. As a result, the application of the second system is limited to low speed machines or copiers and printers which are expected to produce only a small amount of copies or printings, due to limitations relating to mounting and a toner storing capacity.
The third system is capable of conveying toner stored or collected by, e.g., cleaning via a flexible piping or similar piping. Therefore, sure toner conveyance is achievable with a simple configuration without any limitation relating to a toner storing capacity or the location of a toner collecting section. Further, because the screw pump conveys toner in the form of a toner and air mixture, the toner can be stably conveyed while the toner being conveyed is free from undesirable stresses and prevented from cohering or melting.
In the third system advantageous over the first and second systems for the above reasons, the screw pump or so-called Mono pump is made up of a female screw type stator and a male screw type rotor rotatably received in the stator. The stator is formed with a double pitch spiral groove in its inner periphery.
The rotor of the screw pump is formed of rubber or similar elastic material. Therefore, while the rotor repeatedly rotates in sliding contact with the inner periphery of the stator, the inside diameter of the stator increases little by little. As the amount of bite between the stator and the rotor decreases due to the decrease in the inside diameter of the stator, air fed to the outlet side of the screw pump is apt to flow reversely toward the inlet side of the pump via the resulting gap between the stator and the rotor. Because the reverse flow of air effects the conveyance of powder, the screw pump is determined to have reached the end of its life and is replaced with a new screw pump. Of course, the screw pump with such a limited life is operable over only a relatively short period of time. To extend the life of the screw pump, the initial amount of bite between the stator and rotor may be increased such that an expected amount of bite is guaranteed over a long period of time despite the wear of the stator. This, however, brings about a problem that the toner is apt to cohere during conveyance.